MU-MIMO is a challenging technology to implement correctly and won’t be available in the first wave of AP products. And even when available, MU-MIMO does come with caveats.
Figure 8 shows one piece of the puzzle. To send data to user 1, the AP forms a strong beam toward user 1, shown as the top right lobe of the blue curve. At the same time the AP minimizes the energy for user 1 in the direction of user 2 and user 3. This is called “null steering” and is shown as the blue notches. In addition, the AP is sending data to user 2, forms a beam toward user 2, and forms notches toward users 1 and 3, as shown by the red curve. The yellow curve shows a similar beam toward user 3 and nulls toward users 1 and 2. In this way, each of users 1, 2, and 3 receives a strong copy of the desired data that is only slightly degraded by interference from data for the other users.
For all this to work properly, especially the deep nulls, the AP has to know the wireless channel from itself to all of the users very accurately. And since the channel changes over time, the AP has to keep measuring the channel, which adds overhead. Some APs might use the higher-overhead 802.11ac sounding protocol only, but the greatest benefit of MU-MIMO comes if the AP can minimize the number of explicit sounding exchanges, such as with the ClientLink mechanisms.
Meanwhile, the client is receiving its desired signal distorted by some interference from the signals intended for other users. This interference makes the highest constellations such as 256QAM infeasible within an MU-MIMO transmission.
In summary, MU-MIMO allows an AP to deliver appreciably more data to its associated clients, especially for small‑form-factor clients (often BYOD clients) that are limited to a single antenna. If the AP is transmitting to two or three clients, the effective speed increase varies from a factor of unity[3] (no speed increase) up to a factor of two or three times, according to wireless channel conditions.
SISO, SIMO, MISO, MIMO terminology
The different forms of antenna technology refer to single or multiple inputs and outputs. These are related to the radio link. In this way the input is the transmitter as it transmits into the link or signal path, and the output is the receiver. It is at the output of the wireless link.
therefore the different forms of single / multiple antenna links are defined as below:
- SISO - Single Input Single Output
- SIMO - Single Input Multiple output
- MISO - Multiple Input Single Output
- MIMO - Multiple Input multiple Output
The term MU-MIMO is also used for a multiple user version of MIMO as described below.
MIMO - SISO
The simplest form of radio link can be defined in MIMO terms as SISO - Single Input Single Output. This is effectively a standard radio channel - this transmitter operates with one antenna as does the receiver. There is no diversity and no additional processing required.
SISO - Single Input Single Output
The advantage of a SIS system is its simplicity. SISO requires no processing in terms of the various forms of diversity that may be used. However the SISO channel is limited in its performance. Interference and fading will impact the system more than a MIMO system using some form of diversity, and the channel bandwidth is limited by Shannon's law - the throughput being dependent upon the channel bandwidth and the signal to noise ratio.
MIMO - SIMO
The SIMO or Single Input Multiple Output version of MIMO occurs where the transmitter has a single antenna and the receiver has multiple antennas. This is also known as receive diversity. It is often used to enable a receiver system that receives signals from a number of independent sources to combat the effects of fading. It has been used for many years with short wave listening / receiving stations to combat the effects of ionospheric fading and interference.
SIMO - Single Input Multiple Output
SIMO has the advantage that it is relatively easy to implement although it does have some disadvantages in that the processing is required in the receiver. The use of SIMO may be quite acceptable in many applications, but where the receiver is located in a mobile device such as a cellphone handset, the levels of processing may be limited by size, cost and battery drain.
There are two forms of SIMO that can be used:
- Switched diversity SIMO: This form of SIMO looks for the strongest signal and switches to that antenna.
- Maximum ratio combining SIMO: This form of SIMO takes both signals and sums them to give the a combination. In this way, the signals from both antennas contribute to the overall signal.
MIMO - MISO
MISO is also termed transmit diversity. In this case, the same data is transmitted redundantly from the two transmitter antennas. The receiver is then able to receive the optimum signal which it can then use to receive extract the required data.
MISO - Multiple Input Single Output
The advantage of using MISO is that the multiple antennas and the redundancy coding / processing is moved from the receiver to the transmitter. In instances such as cellphone UEs, this can be a significant advantage in terms of space for the antennas and reducing the level of processing required in the receiver for the redundancy coding. This has a positive impact on size, cost and battery life as the lower level of processing requires less battery consumption.
MIMO
Where there are more than one antenna at either end of the radio link, this is termed MIMO - Multiple Input Multiple Output. MIMO can be used to provide improvements in both channel robustness as well as channel throughput.
MIMO - Multiple Input Multiple Output
In order to be able to benefit from MIMO fully it is necessary to be able to utilise coding on the channels to separate the data from the different paths. This requires processing, but provides additional channel robustness / data throughput capacity.
